With the development of industries, damages due to particulate and gaseous contaminants discharged from industrial processes are becoming more serious. Therefore, in order to prevent the discharge of particulate contaminants contained in exhaust gas, filters have been generally used. However, among these filters, polymer filters have drawbacks in heat resistance, chemical resistance, wear resistance, and flame resistance. For example, polyester filters have a problem that they undergo contraction at 150° C., and PTFE (Teflon) filters having excellent heat resistance also have a problem that they cannot tolerate a temperature of 300° C. or higher. Further, a process using an industrial filter has a problem that a large amount of particulate contaminants having strong wear properties are discharged, and these discharged particulate contaminants damage the surface of the filter material, such as polyester, polypropylene, acryl, polyamide, polyimide, glass fiber or the like, thus reducing the lifetime of the filter. In addition, there are also problems that fires may occur due to the generation of flames during a combustion process, or the filtering effect of exhaust gas is reduced due to the holes generated in the filter.
Accordingly, in order to solve such problems, ceramic filters have been developed. Ceramic filters are characterized in that they have very excellent heat resistance, chemical resistance and wear resistance compared to polymer filters, and are advantageous in that they have excellent heat resistance, not necessitating a cooling apparatus or the like to be additionally installed in an exhaust system, thus reducing installation and maintenance costs.
In the case of existing ceramic filters, generally, they are manufactured by a method including making a composition into slurry; and vacuum-forming or extrusion-forming the slurry into a tube. However, this method is disadvantageous in that it is difficult to freely control filtration efficiency and pressure loss, in that the manufacturing cost of the ceramic filter is high, and in that, when the ceramic filter is used for a long period of time, dusts accumulate in the ceramic filter, thus reducing filtration performance. Further, they have a problem that, when dust is shaken off from the outer wall of the ceramic filter by reversely injecting compressed air at the time of regeneration of the ceramic filter, the ceramic filter is damaged, and, at the time of normal operation using this damaged ceramic filter, dusts are included in exhaust gas and then discharged, thereby causing a secondary pollution. In addition, there is a problem that, since the ceramic filter is configured to have uniform porosity in the width direction thereof (inner and outer walls), a clogging phenomenon occurs when dusts are collected in the ceramic filter, thus increasing pressure loss.
Further, in terms of a manufacturing process, a vacuum process is problematic in that it takes a high cost to fabricate a vacuum chamber and a vacuum pump, and the size of a manufacturable ceramic filter is restricted depending on the restriction of size of a vacuum chamber, and thus a large-size ceramic filter cannot be manufactured, and in that a ceramic material is seriously worn, and thus a mold must be replaced with respect to each predetermined production amount, thereby increasing a production cost.
Meanwhile, in addition to the vacuum forming, extrusion forming, press forming, hydrostatic pressure forming, etc., have been developed. However, even in this case, there is a problem in that a mold must be fabricated and a pressure device must be provided, so a manufacturing cost increases, a manufacturing process is not easy, the change of a manufacturing shape is not easy when the mold is determined, and the production of a large-size filter is difficult. Further, in the case of the above forming methods, there is a problem in that, since theses forming methods are a pressure forming method, air permeability is lowered due to low porosity, and pressure loss increases at the time of passing through a filter.
Thus, the present inventors, while endeavoring to find a method of manufacturing a ceramic filter for hot gas filtration capable of easily adjusting a pore size and being manufactured at low cost and low energy, discovered that a ceramic filter for hot gas filtration, manufactured by aerosolizing a ceramic composition powder through a drying process instead of a wet process, exhibits excellent filter characteristics, and can overcome the above problems, thereby completing the present invention.